Bateman identified two aspects of sexual selection. The first, called Bateman's principle, is that sexual selection favors increased promiscuity of males but not of females as a result of differences in parental investment in gametes. The second is that the variance in mate number of males is the fundamental cause of a sex difference in fitness variance. We argue that Bateman's insight about the source of sexual selection is more fundamental than his speculation about patterns of parental investment. We show that, when the sex ratio is 1:1, the average female must be as promiscuous as the average male, because each copulation involves one male and one female. Because mean male and female promiscuity are tied together in the same manner as mean male and female fitness, a sex difference in mating propensity must be the result of either (1) a sex difference in the covariance between matings and number offspring, or (2) Fisherian run-away sexual selection, wherein female reluctance to mate is a weak form of female choice. We show how female promiscuity can limit the evolution of male promiscuity, turning the central argument of parental investment theory on its head.